Adds default devcontainer configuration from SSWG:
https://github.com/swift-server/swift-devcontainer-template
### Motivation:
Developing for linux can be challenging on macOS because some APIs are
not available on linux. By using VSCode and devcontainer you can
essentially develop "on linux".
### Modifications:
Adds devcontainer config.
### Result:
When working in VSCode, you can now easily open the project inside a
devcontainer.
Tobias